Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration. Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device downhole. For example, it is often useful to determine the viscosity of formation fluids, including those that occupy pore space in the formation, since the knowledge of whether light or heavy oils are present can influence the course of oil field production operations.